The invention relates to a sun search method and apparatus for a satellite which is stabilized in three axes.
International patent document WO 93/04923 A1 discloses a measuring device of this generic type for use in attitude regulation of a satellite stabilized in three axes, and a method for acquiring the sun from any initial attitude of the satellite in which the sun is not within the visual field of the sun sensors. This measuring system comprises sun sensors to determine the direction of the sun relative to a coordinate system which is integral with the satellite as well as a rotational speed gyro which measures on one axis. The visual field and/or measuring range of the sun sensors must cover the full angle of 2.pi. in a preselectable plane of the coordinate system, and only one limited angle range less than .pi./2 in both directions orthogonal thereto. The permissible direction of the measuring axis of the rotational speed gyro is thus subject to a limitation that depends on the width of the visual field of the sun sensors orthogonal to the preselectable plane. The conditions specified for the angles .alpha..sub.1 and .alpha..sub.2 that define the visual field of the sun sensors in the preselectable plane and orthogonal thereto, and for the permissible range of angle .beta. measured between the measuring axis of the rotational speed gyro and the preselectable plane are defined therein as follows: EQU 0.ltoreq..alpha..ltoreq.2.pi. (1a) EQU -.alpha..sub.2max .ltoreq..alpha..sub.2 .ltoreq..alpha..sub.2max( 1b) EQU .vertline..beta..vertline..gtoreq.(.pi./2) -.alpha..sub.2max( 1c)
The XZ plane of the satellite-integral coordinate system is preferred as the preselectable plane that must be completely covered by the visual field of the sun sensor system, with the Z axis being oriented to the center of the earth as the yaw axis in the case of a geostationary earth satellite for example, the X axis aligned in the direction of the orbit as the roll axis, and the Y axis, being orthogonal to the other two axes, as the pitch axis.
However, it may happen that because of the requirements imposed by other operational maneuvers, the above condition relative to the direction of the measuring axis of the rotational speed gyro can no longer be fulfilled; i.e., EQU .beta..sub.max .ltoreq.(.pi./2) -.alpha..sub.2max, (2)
In addition, the above requirement that the visual field of the sun sensor system in the preselectable plane must include the full angle of 2.pi. constitutes a limitation that cannot always be allowed. Also, it is also not optimal for cost reasons, since a corresponding number of sun sensors must be provided to cover the entire angle range of 2.pi.. In addition, if one or more sun sensors should fail, gaps will occur in the visual field, resulting in the provided measurement and evaluation method and in particular the provided sun acquisition method no longer being operable. The same is true also if the all-around visual field is restricted by projecting antennas or other devices mounted on the satellite.
The goal of the present invention is to provide a method and apparatus of the species recited at the outset for sun acquisition for a satellite stabilized in three axes, that functions even when the limitations defined above regarding the sun sensor visual field as well as the direction of the measuring axis of the rotational speed gyro can no longer be maintained.
Another object is to provide a method and apparatus for sun acquisition which is usable even when the visual field of the sun sensor system has gaps in the plane and the above-mentioned measuring axis is oriented arbitrarily with respect to this visual field.
Finally, still another object of the invention is to provide a satellite stabilized in three axes which utilizes the sun search method and apparatus according to the invention.
The goal regarding the sun search method is achieved by the sun acquisition method and apparatus according to the invention, in which a flywheel device is used to generate a momentum whose corresponding momentum vector H need not be oriented parallel to the direction of the measuring axis of the rotational speed gyro. The use of a known regulating procedure (e.g., WO 93/04923 A1) is assumed in this connection, making the rotational speed component oriented parallel to the measuring axis of the rotational speed gyro equal to zero. Generation of the additional momentum means that as a result of the regulating process, merely a rotation of the satellite around the momentum axis remains. This revolution will in a great many cases suffice for the sun to appear in the visual field of the sun sensor system.
In an advantageous embodiment of the invention, the visual field of the sun sensor system continuously covers an angle of at least .pi. in the preselectable plane, while in another embodiment, the visual field has one or more sectors interrupted by gaps, none of which covers an angle of .pi..
According to the invention, a satellite can be stabilized in three axes without the limitations recited at the outset regarding its sun sensor system as well as the direction of the measuring axis of its rotational speed gyro, measuring on one axis. In other words there is no longer all-around visibility of the sun sensor system in the preselectable plane, and the single measuring axis of the rotational speed gyro can be oriented as desired. The important feature is the addition of a flywheel device capable of generating momentum components around all three coordinate axes. Such a satellite with only one uniaxially measuring rotational speed gyro when the sun sensor visual field is sharply restricted for rotational speed measurement, could not have been designed earlier because of the problem that a sun search must be commenced from an unknown initial attitude. The problem could be solved in the past only when a rotational speed gyro was available that measured on three axes and, especially in the case of a redundant design, was correspondingly expensive and delicate. In contrast, in the method and apparatus according to the invention, a satellite uses a uniaxially measuring rotational speed gyro for the essential sun search maneuver, which is made possible by the flywheel device that is provided.
Objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.